Optical disks record and reproduce signals by altering the reflection properties of a substrate on a recording-film. Generally, a signal is recorded as minute spots on a substrate having optical spot diameters which are on the order of the light wavelength used during reproduction or play-back. Such media are used to produce optical disks such as compact disks (CDs) and a laser video disks (LVDs or DVDs). There are a variety of materials and methods used for such recording media, resulting in media that are read-only, media that are record once/read only, and media that may repeatedly be recorded, read and erased.
In a read only type optical disk, a signal track is recorded as a series of small concave and convex pits between a transparent disk substrate such as resin or the like and a reflective film comprising aluminum or similar materials. During signal reproduction, a scanning operation is effected on the signal track by focusing a laser beam on the series of pits. The laser beam is reflected at locations where pits do not exist and is substantially diffracted at locations where pits exist. By using an optical detector, the location of the signal pits can be discriminated in accordance with a magnitude of the reflected light, and interpreted as a digital signal of ones and zeros.
Media that can be recorded during use employ materials whose optical properties are switchable between at least two detectable states by the application of optical energy during the recording process. The state of the phase-change material is detectable by properties such as, for example, index of refraction, optical absorption, optical reflectivity, or combinations thereof. The media is usually comprised of a transparent substrate, an undercoat, an active layer and an overcoat. The active is a chalcogenide phase change material that has a crystalline state and an amorphous state, or that has two crystal states. Initially, the active layer is in the crystalline state. In the former case, the amorphous state is formed by heating a portion or spot of the active layer with a high power laser pulse of short duration to a temperature above its melting point to change it to a liquid state. If the spot cools sufficiently rapidly it changes to the amorphous state. In erasable media, capable of multiple recording cycles, the amorphous material returns to its crystalline state when the amorphous spot is heated again, with a lower powered laser. To read the media, a very low power laser is reflected off of the active layer. The crystalline state has a higher or different reflectivity than the amorphous state and this difference in reflectivity is detected and interpreted as data ones and zeros.
As a recording film material, phase change materials known in the art include those comprising Te, In, Sb, Se or the like as principal components. For example, the thin films containing Te85Ge15, Te87Ge8Sn5, Te92Ge2As5, and Te81Ge15S2Sb2 are known to produce reversible phase-transitions. Such materials are discussed in the following references: U.S. Pat. No. 3,530,441 Ovsinsky, issued Sep. 22, 1970; A. W. Smith, Applied Physics Letters, 18 (1971) p. 254; and M. Chen et al., Applied Physics Letters, 46 (1985) p. 734.
Optical recording discs have been standardized internationally to be a diameter of 120 mm; therefore the only way to increase storage capacity is to increase the density of data storage on the standardized disc. Increased data storage density requires, in turn, smaller individual optical spot diameters. A limitation in this regard is the frequency of light used to form and resolve the individual spots—smaller spots require the use of shorter wavelength (higher frequency) light. The current art responds well to laser pulses in the near-infrared (>1000 nm wavelength) range. What is needed is a material highly responsive in the UV-Vis region greatly increasing the potential capacity of a corresponding storage medium.